responder.rb

class APIResponder < ActionController::Responder

  private

  def display(resource, options = {})
    super(resource.data, options)
  end

  def has_errors?
    !resource.success?
  end

  def json_resource_errors
    { error: resource.error, message: resource.error_message, code: resource.code, details: resource.details }
  end

  def api_location
    nil
  end
end

class Success
  attr_reader :data
  def initialize(data)
    @data = data
  end

  def success?
    true
  end
end

class Error
  attr_reader :error, :code, :details
  def initialize(error = nil, code = nil, details = nil)
    @error = error
    @code = code
    @details = details
  end

  def error_message
    error.to_s
  end

  def success?
    false
  end
end

class ValidationError < Error
  def initialize(details)
    super(:validation_failed, 101, details)
  end

  def error_message
    "Validation failed: see details"
  end
end

class InvoicesController < ApplicationController
  respond_to :json

  def create
    form = InvoiceForm.new(params)
    result = CreateInvoice.new(current_user, form).call # => ValidationError.new(invoice.errors)
    respond_with result # { error: "validation_error", code: 101, message: "..." details: { ... } }
  end


  def update
    result = UpdateInvoice.new(current_user, params[:id]).call # => Success.new(invoice)
    respond_with(result) # { billing_date: ..., company_name: ... }
  end

  # ...

  def self.responder
    APIResponder
  end
end